Over the years, telecommunication systems have achieved a sterling reputation for reliability. One reason for that reputation is that, when other utilities (such as electricity and gas) are interrupted, one's telephone still works. This is due in part to the fact that wire-based telecommunications equipment is powered from a central location equipped with a battery backed-up power unit. The battery backed-up power unit supplies electricity to the equipment when external power is interrupted.
Commercially-available battery backed-up power units typically employ a plurality of batteries, not unlike the lead-acid batteries found in automobiles. The batteries are joined in series to form what is called a "string" of batteries. The strings are sometimes joined in parallel to provide adequate power reserve to the equipment.
By experience, it has been found highly undesirable to allow the batteries in the battery strings to discharge completely. When a battery of the type employed in such units is completely discharged, it becomes impossible to recharge, and the batteries must be discarded. This is both expensive and wasteful. It is far better to discharge a battery string only as far as possible without permanently damaging it and then to disconnect the battery string to prevent it from being discharged further. Accordingly, commercially-available battery backed-up power units frequently provide a low voltage disconnect ("LVD") that detects the output voltage of each battery string or group of parallel strings and, in response thereto, decouples battery strings as their voltage drops below a predetermined minimum level.
The LVD function is conventionally centralized and consists of two separate parts: one or more distributed contactors and central sense and control circuitry. Unfortunately, if a conventional, centralized LVD indicates a failure, it may be difficult to determine whether one of the distributed contactors or the central sense and control circuitry has failed. Given this, manufacturers of such battery backed-up power units have directed that both the distributed contactors and the central sense and control circuitry be replaced when a failure is indicated.
Frequently, however, operators in the field ignored the manufacturers' informed directions, replacing only one of either a contactor or the central control board. As a result, the operators replaced contactors when the fault was in the central control board or replaced the central control board when a contactor was at fault. Without both in working order, the LVD cannot operate properly, exposing at least one of the battery strings to destruction from excessive discharge or erroneously disconnecting one of the battery strings under normal battery voltages, introducing the possibility of loss of telecommunications services during power outages and associated liabilities and loss of revenue.
Accordingly, what is needed in the art is a more effective way of ensuring that a battery backed-up power unit does not lose its LVD function because of poor field maintenance practice.